Wire bonding chips to metal leadframes coated with silver is widely practiced in the semiconductor industry. The silver offers a surface capable of forming a reliable bond. Applicants have identified three problems in prior art approaches that cause fails either on the production line or out in the field: (1) in the presence of moisture, silver tends to migrate across an insulator separating conductors held at different potentials, shorting the conductors; (2) wire bond wires crossing over exposed conductors of the leadframe may short to those conductors; and (3) plastic compounds used to seal the package often do not adhere well to silver.
Various methods have been disclosed in the prior art that address these problems individually. U.S. Pat. No. 4,883,774 to Djennas et al. (the '774 patent) teaches the use of electroless plating to produce a very thin and uniform silver layer across the entire surface of the leadframe. The '774 patent teaches that uniformly deposited silver enhances adhesion between leadframe and plastic encapsulant. The '774 patent also teaches that uniformly deposited silver avoids material gradients that cause silver diffusion. However, it is well known that uniform deposition does not prevent the above-mentioned form of silver migration across an insulator (that is, migration caused by an applied voltage between conductors in the presence of moisture). This migration is not driven by material gradients; it is caused by drift of ions in an electric field, as described in a paper "Metallic Electromigration Phenomena," by S. J. Krumbein, IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 11, No. 1, March, 1988, pp. 5-15.
Shorting between wire and leadframe fingers has been addressed in a technical disclosure "Thin Small Outline Packages," by E. J. Dombroski et al., IBM Technical Disclosure Bulletin, Vol. 34, No. 1, June 1991, pp. 358-359. Here screened and punched tape coatings at wire crossing locations have been used for improved isolation of wire and lead. However, this requires additional processing and cost specifically to achieve improved isolation, without addressing the two other above-mentioned concerns.
The present invention efficiently solves all three problems, providing reduced silver migration between leadframe fingers held at different potentials, improved isolation between leadframe and wire-bond wires, and improved adhesion between plastic encapsulant and leadframe.